It is well known in the agriculture to use a monitor on planters to monitor the seed at each row unit. When first employed, monitors were used to alert the operator of a plugged row unit or a unit without any seed to avoid continued operation of the planter without actually planting seed. More recently, studies have quantified the importance of accurate seed spacing in producing enhanced crop yields. As a result, monitor technology has advanced in efforts to determine seed spacing. Current monitors use the time interval between seeds to determine skips or multiples of seed. These monitors also predict seed spacing in the furrow based on the timing of seed passing the monitor in the seed tube.
A paper entitled Opto-electronic Sensor System for Rapid Evaluation of Planter Seed Spacing Uniformity, Transactions of the ASAE 41(1):237-245 describes using the seed trajectory, speed of the planter and timing of seed release events to determine seed spacing. The goal of the study was to evaluate a sensor located just above the soil surface at the seed drop zone in measuring the seed location relative to the planter. The sensor was then used to determine seed spacing instead of dropping seed onto a grease belt and manually evaluating seed spacing. The sensor had two arrays of 12 pairs of LEDs and photo-transistors to sense and locate the seed along one axis.